Most all communications cables, both copper and glass, installed in domestic communication systems are buried underground. Because of the antagonistic environment encountered, water being the chief concern, these cables are waterproofed. U.S. Pat. No. 4,351,913 and 4,176,240, the contents of which are incorporated herein by references as if faithfully reproduced, report that attempts to waterproof buried cable began nearly 100 hundred years ago, but were not successful in the practical sense until the production of plastic insulated cable (PIC) during the 1950's. It was general practice, where water was a problem, to pressurize the interior of the cable. Although this practice was successful in excluding water from the cable interiors, pressurized cables are expensive to maintain and for this reason have fallen from general use.
Unpressurized unfilled PIC cables fail to solve the water problem because water migrates through the plastic jacket into the interior of the cable and disrupts or deteriorates communication service. Water can also penetrate a PIC sheath through a localized opening and then is able to follow any channel inside the cable as far as physical forces will allow, often hundreds of feet, to ultimately accumulate and flood a local segment. This water upsets the capacitance balance of electrical transmission lines and introduces potential corrosion, which after extended time, ends to deteriorate the useful life of the water-soaked transmission medium. Water flooding of a cable containing optical waveguides can be deleterious to optical transmission, especially when there is alternate freezing and thawing which exacerbate any minute pre-existing cracks.
One widely adopted solution to stop the entry and migration of water in a communications cable is to fill the interstices within the cable with a water-insoluble filling material having the propensity to plug the cable. It has been said many times and recently repeated in U.S. Pat. Nos. 4,176,240 and 4,351,913, that the physical function of filling a cable with filling material is obvious, but the selection of the particular filling material is not. In the selection process, one must consider the hydrophobic nature of the materials, stability in aging, low and high temperature properties, processing characteristics, handling characteristics, dielectric properties, shrinkage, toxicity and cost, just to name the important ones.
One of the challenges facing present day design of cable is to find a suitable filling material with a melting point above 75.degree. C. This problem was identified and only partially faced in U.S. Pat. No. 4,351,913. The majority of the compositions identified and disclosed in this patent indicated a drip temperature of no greater than 75.degree. C. In U.S. Pat. No. 4,176,240, a flow point or drip point of a maximum of 70.degree. C. was achieved by the disclosed filling material. The forementioned patents, along with U.S. Pat. No. 4,324,453 represent the state of the known prior art relative to the instant invention.
U.S. Pat. No. 4,351,913 discloses a mixture of a block copolymer dissolved in a paraffinic or napathenic mineral oil, mixed with an inorganic (glass or ceramic) hollow microspheres plus an additive of either low molecular weight polyethylene or glycerol hydroxy stearate.
The present invention is an improvement over this prior art, the various ingredients employed being as follows:
(a) Block Copolymer: Styrene-ethylene butylene-styrene (SEBS) having a styrene to rubber ratio of 0.39 to 0.41, and a specific gravity of approximately 0.91. Such preferred SEBS block copolymers are available from Shell Chemical Company, Houston, Texas, under a trade designations Kraton G-1650 and G-1652. PA1 (b) Petrolatum: a mixture of microcrystalline waxes and oil. Preferably the amount of oil in the petrolatum used with the instant invention is no more than 15 percent as determined by ASTM D 721. Such a material can be procured from Penreco, Inc. of Butler, Pa. However, all so-called cable grade petrolatums are deemed to be operable. A typical petrolatum used had a nominal melting point of 57.2.degree. C., density of 0.88 grams/cm.sup.3 at room temperature, oil content of no more than 15 weight percent, dielectric constant of 2.25 maximum at 10.sup.5 to 10.sup.6 Hertz and dissipation factor of 0.0004 maximum at 10.sup.5 Hertz and 0.0008 maximum at 10.sup.6 Hertz. It also contained a small amount of an antioxidant additive, namely 0.2 weight percent Irganox-1030, available from Ciba-Geigy, Ardsley, N.Y. PA1 (c) Additive: a low molecular weight polyethylene having a molecular weight range from 1,000 to 10,000 and a specific gravity of at least 0.90. A preferred polyethylene, as used in the present invention, has a specific gravity from 0.93 to 0.94. A polyethylene of this nature is manufactured by the Allied Chemical Company of Morristown, N.J. and sold under the mark "AC-8." Other low molecular weight polyethylenes are also operable.
The block copolymers and polyethylene are dissolved in the petrolatum. The amounts of the ingredients described below have been found to give a cable filling material that meets the functional requirements of the cable technology, have handling characteristics superior to those of the prior art materials, and in most cases a melting point in excess of 75.degree. C.